This invention relates to a solid film of iron(III) hexacyanoferrate(II) deposited upon a electronically conductive surface and the process for synthesizing same.
The iron(III) hexacyanoferrate(II) used herein is a blue pigment which is generally known as Prussian blue.
It is well known from the past that the mixing of an aqueous solution containing Fe(II)(CN).sub.6.sup.4- or Fe(III)(CN).sub.6.sup.3- results in a variety of precipitates as shown in Table 1. These reactions have been widely utilized for the qualitative analysis of the ions referred to in Table 1. The Prussian blue shown in Table 1 has been widely used as a blue pigment for printing ink and the like since its discovery in 1704.
Table 1 is a list of the materials which result when aqueous solutions containing Fe(II) or Fe(III) ion are mixed with aqueous solutions containing Fe(II)(CN).sub.6 !.sup.4- or Fe(III)(CN).sub.6 !.sup.3- ion.
TABLE 1 ______________________________________ Cyanoferrate ion Ferrocyanide ion Ferricyanide ion Iron ion Fe(II)(CN).sub.6 !.sup.4- Fe(III)(CN).sub.6 !.sup.3- ______________________________________ ferrous ion Everitt's salt Turnbull's blue Fe(II) K.sub.2 Fe(II)Fe(II)(CN).sub.6 ! KFe(II)Fe(III)(CN).sub.6 ! white precipitate blue precipitate ferric ion Prussian blue Prussian brown Fe(III) KFe(III)Fe(II)(CN).sub.6 ! or Fe(III).sub.4 Fe(II)(CN).sub.6 !.sub.3 blue precipitate clear brown solution ______________________________________
In Table 1, Prussian blue and Turnbull's blue which had been considered to be different compounds were recently identified as a single chemical compound by a variety of physicochemical measurements, including Mossbauer spectroscopy and absorption spectroscopy.
Among the prior art processes known for the synthesis of Prussian blue have been known comprising mixing an iron(III) ion-containing solution with a hexacyanoferrate(II) ion-containing solution or alternatively mixing an iron(Ii) ion-containing solution with a hexacyanoferrate(III) ion-containing solution, and causing Prussian blue to deposit as an insoluble blue precipitate from the solution mixture. The iron(III) ion-containing solution used in such a synthesizing process is, for example, an aqueous solution of iron(III) chloride or iron(III) sulfate, and the hexacyanoferrate(II) ion-containing solution used is, for example, an aqueous solution of potassium hexacyanoferrate(II) or sodium hexacyanoferrate(II). In the alternative process an iron(II) ion-containing solution and a hexacyanoferrate(III) ion-containing solution are used in combination, examples of the former are aqueous solutions of iron(II) chloride and iron(II) sulfate, and an examples of the latter are aqueous solutions of potassium hexacyanoferrate(III) and sodium hexacyanoferrate(III). Whichever combination is used in these prior art processes, it is very difficult to externally control the rate of reaction to a desired level in synthesizing iron(III) hexacyanoferrate(II), because as soon as two different solutions are mixed, chemical reaction takes place very rapidly in the solution mixture. Further, it is impossible to deposit and form a thin film of iron(III) hexacyranoferrate(II) complex directly on a substrate, because the reaction of producing iron(III) hexacyanoferrate(II) occurrs simultaneously throughout the solution mixture to form and precipitate insoluble particles of the three-dimensional complex. While it is possible to form an iron(III) hexacyanoferrate(II)-containing coating on a substrate by applying a dispersion of Prussian blue pigment in a suitable paint on the substrate, no process has been available for forming a thin film of iron(III) hexacyanoferrate(II) directly on a substrate.